1. Field of the Invention
The present invention relates to polarizing glasses employed in optical products such as optical isolators, and more particularly, to high performance polarizing glasses comprising geometrically anisotropic metallic silver particles. The present invention further relates to methods of manufacturing these polarizing glasses.
2. Description of Related Art
Polarizing glasses comprising geometrically anisotropic metallic silver particles can be manufactured by the methods described in Japanese Patent Application Examined Publication No. Hei 2-40619 (Referenced publication 1) and Japanese Patent Application Un-examined Publication No. Sho 56-169140 (Referenced publication 2).
In these methods, glass comprising a silver halide is heat-treated to deposit out the silver halide and the glass is elongated to lengthen the silver halide particles. The glass is then heat treated in a reducing environment to reduce the silver halide particles to silver, thereby manufacturing polarizing glass comprising geometrically anisotropic silver particles.
For example, the following method is specifically described in Referenced publication 1.
A method of manufacturing glass articles exhibiting excellent polarization in the infrared region of the spectrum from glasses containing silver halide particles therein selected from the group of AgCl, AgBr, and AgI, characterized by comprising steps in which (a) a batch for a glass containing silver and at least one halide selected from the group of chloride, bromide, and iodide is melted and the melt shaped into a glass body of a desired geometry; (b) the glass body is subjected to a heat treatment at least above the strain point but not in excess of 50xc2x0 C. above the softening point of the glass for a period of time adequate to cause the generation of AgCl and/or AgBr and/or AgI particles therein, said particles ranging in size between 200-5000 xc3x85; (c) the glass body is elongated under stress at a temperature above the annealing point but below that where said glass exhibits a viscosity of about 108 poises, such that the particles are elongated to an aspect ratio of at least 5:1; and (d) the elongated glass body is exposed to a reducing environment at a temperature between 250xc2x0 C. and about 25xc2x0 C. above the annealing point of the glass to reduce at least a portion of the silver halide particles in the glass to silver particles which is deposited in or on the elongated particles.
Referenced publications 1 and 2 disclose glasses employed in polarizing glass, for example, exhibiting photochromic characteristics and having a composition consisting essentially, expressed in terms of weight percent on the oxide basis, of 6-20 percent R2O (where R2O consists of 0-2.5 percent Li2O, 0-9 percent Na2O, 0-17 percent K2O, and 0-6 percent Cs2O), 14-23 percent B2O3, 5-25 percent Al2O3, 0-25 percent P2O5, 20-65 percent SiO2, 0.004-0.02 percent CuO, 0.15-1.3 percent Ag, 0.1-0.25 percent Cl, and 0.1-0.2 percent Br, the molar ratio R2O:B2O3 ranging between about 0.55-0.85 when the composition is essentially free from divalent metal oxides other than CuO, and the weight ratio Ag:(Cl+Br) ranging between about 0.65-0.95.
In such types of polarizing glasses, the silver halide that is reduced in the silver halide crystal reducing step is just the outer layer portion, with silver halide crystals being present in the glass matrix in large quantity. When the silver halide exhibits photochromic characteristics, exposure to ultraviolet or visible light causes darkening and absorption of near infrared light, compromising the polarization characteristics of the polarizing glass, and in particular, causing a significant transmission loss.
Thus, Referenced publication 1 discloses a molar ratio of (R2Oxe2x80x94Al2O3):B2O3 less than 0.25 and essentially the absence of CuO in the above-recited composition exhibiting photochromic characteristics as a composition rendering polarizing glass non-photochromic.
Japanese Patent No. 2628014 (Referenced publication 3) discloses another type of non-photochromic polarizing glass. Referenced publication 3 points out the problem in Referenced publication 1 that, in the glass batch melt or during the heat treatment generating silver halide crystals, silver is reduced to a metallic state and silver halide crystals are not generated in the heat treatment the purpose of which is to generate silver halide crystals, and describes a non-photochromic, silver halide-comprising, polarizing glass composition in the form of a composition comprising essentially no silver and a quantity of CeO2 adequate to effectively maintain the silver in the glass in an oxidized state. CeO2 oxidizes silver, and is employed as an oxidizing agent for the silver in place of CuO, which is thought to cause photochromism, thereby preventing the development of photochromic characteristics.
In such polarizing glasses, it is extremely important to stabilize the glass because of the use of a heat treatment step to deposit silver halide. However, the above-recited composition of polarizing glass has drawbacks in that the glass is thermally unstable and the glass loses transparency during the course of the heat treatment, that is, a haze is generated in the glass as the result of the deposition of crystals other than silver halide crystals. As a result, light entering the polarizing glass is scattered and transmission loss increases. In recent years in particular, since higher extinction ratios and lower losses have been demanded of the polarizing glass employed in optical components in the field of optical communications and the like, the increase in transmission loss is a major problem.
Further, in the manufacturing of non-photochromic polarizing glass, the CeO2 employed in Referenced publication 3 has the same oxidizing effect as CuO and effectively prevents the reduction of Ag, as indicated by the formulas given below. However, in methods adding an oxidizing agent such as CuO and CeO2, the quantities added to prevent reduction of Ag in the course of melting the glass must be changed based on the melt environment and melt conditions. Further, Cu2+, Cu+, Ce4+, and Ce3+ ions coexist in the glass. Since the chemical equilibrium of these ions tends to vary with temperature, there is a risk that silver will be reduced to a metallic state in the subsequent heat treatment step used to form silver halides.
2CuO less than = greater than Cu2O+O 2CeO2 less than = greater than Ce2O3+O
Even the further addition of CeO2 does not fully prevent photochromism, but causes nucleation promoting the growth of deposits of undesirable crystals other than Ag halide crystals. This is problematic in that it increases transmission loss.
The present invention, devised in light of the above-described problems, has for its object to provide polarizing glass with low transmission loss and a high extinction ratio. A further object of the present invention is to provide a polarizing glass permitting the reduction of silver without the deposition of metallic silver in the heat treatment step for generating a glass melt and silver halide crystals essentially without the addition of oxidizing agents such as CuO and CeO2.
The composition of photochromic glass is similar to that of the base glass employed in such polarizing glasses. The present inventors are the inventors of record of Japanese Patent Application Examined Publication No. Sho 56-51143 (Referenced Publication 4) disclosing the composition of photochromic glass comprising silver halide crystals for use in eyeglasses.
In glass for use in eyeglass lenses, there is the technical problem of conforming to the standard refractive index (Nd 1.523). Japanese Patent Application Examined Publication No. Sho 56-51143 describes the effectiveness, when incorporating refractive index raising components in the form of TiO2 and ZrO2, of keeping the quantity of TiO2 low and incorporating ZrO2 into a composition with little Al2O3, thereby yielding a thermally stable glass with a low liquidous temperature at a refractive index of 1.5 and above and better photochromic performance and chemical durability than compositions comprising large quantities of Al2O3.
The extinction ratios demanded of polarizing glass in optical components have been steadily increasing in recent years (for example, 40-50 dB or more at the chief wavelengths (center wavelengths of 1.31 xcexcm and 1.55 xcexcm) employed in the field of optical communications). In such high-performance polarizing glasses, the reduction of transmission loss is an extremely important problem. Accordingly, the present inventors employed means such as those described in above-cited Referenced publication 4 in the composition of the base glass of polarizing glass as well, discovering that the thermal stability of the glass was increased relative to the polarizing glasses disclosed in Referenced publications 1 and 2, preventing the scattering of incident light by avoiding the loss of transparency of glass due to heat treatment; that the optical scattering caused by differences in refractive index with silver halide crystals was reduced by increasing the refractive index of the glass; and finally, that when employed as polarizing glass, the transmission loss of incident light could be reduced. The present invention was devised on that basis.
Specifically, the present invention relates to a polarizing glass comprising geometrically anisotropic particles dispersed in an oriented manner in at least the surface of a glass base body, characterized in that the glass base body is denoted by the weight percentages of
50-65 percent SiO2,
15-22 percent B2O3,
0-4 percent Al2O3,
2-8 percent ZrO2,
6 percent  less than Al2O3+ZrO2 less than 12 percent,
6-16 percent R2O (where R denotes at least one from among Li, Na, and K),
0-3 percent Li2O,
0-9 percent Na2O,
4-16 percent K2O,
Li2O+Na2O less than K2O,
0-7 percent BaO and/or SrO, and
0-3 percent TiO2;
by comprising per 100 weight percent of essentially the above composition at least 0.15-1.0 percent Ag and at least the chemical equivalent to Ag of Cl and/or Br; and in that the geometrically anisotropic silver particles are metallic Ag particles.